Thermosiphon systems heat potable water or heat transfer fluid
and use natural convection to transport it from the collector to
storage. With reference to Fig. 1, the thermosiphonic effect occurs
because the density of water (or heat transfer fluid) drops with the
increase of the temperature. By the action of solar radiation
absorbed, the water in the collector is heated and thus expands
becoming less dense and rises through the collector into the top of
the storage tank. There it is replaced by the cooler water that has
sunk to the bottom of the tank, from which it flows down the
collector. Circulation continuous as long as there is sunshine.